Chemical Pressure Effects on the Stokes Shift of Bi³⁺ Luminescence in Orthorhombic Perovskites

The relationship between the Stokes shift of Bi³⁺ emission and the volume of the unit-cell in two series of orthorhombic perovskites, LnB³⁺O₃ (Ln = La, Gd, Y; B³⁺ = Al, In, Ga) and AB⁴⁺O₃ (A = Ca, Sr; B⁴⁺ = Zr, Sn) is explored. The Stokes shift increases linearly with increasing cell volume. This is explained qualitatively by the lattice chemical pressure acting on the Bi³+ ion. The degree of Bi³⁺ ion off-centering displacement, which is due to the stereochemical activity of the lone-pair electrons (6s²), is controlled by the chemical pressure. A small cell suppresses the off-centering displacement to produce a small Stokes shift of emission by limiting the excited state structural distortion. In large cell, the off-centering displacement is more easily accommodated. The elimination of ground state distortion in the excited state gives larger Stokes shift of emission. These qualitative arguments are supplemented by recent first-principles calculations on Bi³⁺ luminescence in these perovskites. The Bi³+ luminescence in SrZrO₃, previously assigned to emission from the D-state, is now assigned to the localized ³P_0,1 → ¹S₀ transition. The energy of the ¹S₀ → ³P₁ transition is correlated with the covalence of the BO_6/2 perovskite framework. Discussion on the effective ionic radius of the Bi³⁺ ion in these perovskites is presented.